Fuel in a turbine engine is initially ignited by a high voltage spark discharge at an igniter. Once a turbine engine is started, combustion normally will continue without further use of the igniter until the engine is stopped. However, under certain conditions the flame can be extinguished. At critical times such as during aircraft takeoffs and landings and during combat for military planes, the igniter is periodically fired, for example at from about 2 to 6 or more firings per second. If the flame should become extinguished during such time, it is immediately re-ignited.
Two similar types of igniter exciter circuits are in common use today. Both require a high voltage storage capacitor and a spark gap in the high voltage output circuit in the exciter box. In a first circuit, the high voltage ac output from a transformer is applied through a diode rectifier to charge the capacitor. Normally a mica storage capacitor is charged to about 3,000 volts. The internal spark gap is connected in series between the storage capacitor and the igniter. When the voltage across the capacitor becomes sufficient to jump the internal spark gap, the energy stored in the capacitor is discharged through the spark gap to the igniter. In a second circuit, the rectified output is applied to a capacitor connected in series with the igniter. A resistor is connected in parallel with the igniter to permit charging of the capacitor since the igniter will be essentially an open circuit until a spark discharge ionizes gases at the igniter's spark gap. A spark gap in the exciter is connected across the rectified high voltage ac output from a transformer. When the charge on the capacitor becomes sufficient to discharge across the internal spark gap, energy stored in the capacitor is discharged to the igniter. In both types of igniter exciters, the internal spark gap and the high voltage storage capacitor are expensive and the spark gap prevents accurately controlling the ignition timing. Variations in the voltage required to discharge across the internal spark gap result in variations in ignition timing between different exciters. Also, the rate at which the capacitor is charged can vary with the voltage from the external power source, which will vary the ignition timing.